The invention concerns a headbox for a machine for the production of fiber material webs, for instance paper webs, and, more particularly, to a headbox having a slot connected to measuring devices determining the local position of the slot. Such headboxes are known from the Voith brochure "Headboxes" No. p 2503. A headbox of that type has two flow guide walls; these converge to each other in flow direction, thus forming (together with side walls) a nozzle type machinewide stock channel which features a machinewide stock outlet gap. At least one of the two flow guide walls is movable or has a movable part, whereby a coarse adjustment of the clearance of the stock outlet gap can be effected.
Additionally available yet is an arrangement for fine adjustment of the outlet gap clearance, namely in the form of a slat which extends on one of the two flow guide walls (for example on the movable flow guide wall) along the stock outlet gap across the entire machine width, and which relative to this flow guide wall is transverse to the direction of outflow adjustable and locally deformable. More exactly: the aforementioned flow guide wall is subdivided in a stiff major part and the adjustable and locally deformable slat. This stiff major part may be a rigid, i.e., immovable part of the headbox or may be fashioned as the aforementioned flow guide wall which is movable relatives to the remaining headbox (or a movable flow guide wall part).
For adjustment of the said slat there are numerous adjustment spindles provided which are arranged distributed across the machine width. Each of these adjustment spindles is individually movable along its axis by means of a spindle drive. Thus, a small local deformation can be imparted to the slat by means of each individual one of the numerous adjustment spindles. Of course, it is also possible to operate several adjacent adjustment spindles simultaneously. The objective is normally to adjust the clearance of the discharge gap as much as possible exactly alike across the machine width so that, as the final result, the paper web produced on the machine will obtain maximally constant propertiesacross the machine width (particularly a constant basis weight). But sometimes it may also be desirable to cause at a specific point of the web width, for instance at the edges, deliberately a variation from the normal web properties by adjustment of individual spindles. Exceptionally it may happen that all of the adjustment spindles are actuated at the same time, in order to adjust the slat uniformly across the entire machine width.
The said slat may be fashioned differently., preferably it is designed as a so-called profile bar which has the shape of a ruler and is arranged on the outlet end of the respective flow guide wall (that is, on the said stiff major part of the flow guide wall). Such a profile bar--viewed in longitudinal section through the headbox--is adjustable transverse to the longitudinal expanse of the flow guide wall and locally deformable.
Another embodiment of the movable slat may be realized in that the outlet end of the respective flow guide wall is by way of a thin spot connected with the stiff major part of the flow guide wall. In this case, the outlet end of the flow guide wall may itself be adjusted or locally deformed.
Besides these known features, the following can be seen yet from the aforementioned Voith brochure: to make it possible--at any time during the operation--to read along the entire length of the slat its local position, there are numerous measuring devices provided which are distributed across the machine width. Illustrated is a preferred embodiment where each of the adjustment spindles features on its end away from the slat such a measuring device. According to photograph 9, the measuring devices are designed as preferably mechanical measured value indicators. According to photograph 5, a so-called "path pickup" is provided additionally on each spindle. This is a measuring device fashioned as an electrical signal emitter. The entirety of these signal emitters serves the remote indication of the position, respectively the state of deformation of the said slat and additionally--as required--the transfer of appropriate data to a process control system which, among others, controls the adjustment spindle drives in a way such that the paper web will obtain the desired basis weight cross profile.
While the prior headboxes have proved themselves in practice, it has nonetheless been found that the accuracy of the measurement of the position, respectively of the state of deformation of the said slat, is with the prior arrangement of the measuring devices not sufficient in all cases.